Optical cable

ABSTRACT

An optical cable according to the present invention relates to an optical cable having a construction to enable reduction of a cable outer diameter, and/or improvement of contained efficiency of coated optical fibers while an increase of transmission loss in each coated optical fiber is suppressed. The optical cable has a loose-tube type of structure constructed by: a tension member; a plurality of tubes stranded together around the tension member; and an outer sheath covering the outer periphery of the plurality of tubes. One or more coated optical fibers are contained in each tube. A ratio of A/B is 6.3 or more but 7.0 or less, where each coated optical fiber has a mode field diameter A in a range of 8.6±0.4 μm at a wavelength of 1.31 μm, and where a fiber cutoff wavelength thereof is B μm.

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP2005/020707, filed Nov. 11, 2005,which in turn claims the benefit of Japanese Application No.2004-327950, filed Nov. 11, 2004, the disclosures of which Applicationsare incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to an optical cable constructed bybundling a plurality of optical fibers.

BACKGROUND ART

For the structure of an optical cable bundled by a plurality of coatedoptical fibers, a variety of structures such as tape-slotted type andloose-tube type are known (see Non-Patent Document 1). A tape-slottedtype of optical cable is excellent in contained efficiency of the coatedoptical fiber; there occurs easily bending in the coated optical fiberduring the manufacture or using, which tends to increase micro-bendingloss. On the other hand, a loose-tube type of optical cable ischaracterized in that an increase of the micro-bending loss is smallthough it is inferior to the contained efficiency of the coated opticalfibers as compared to the tape-slotted type of optical cable.

-   Non-Patent Document 1: Gunther Mahike, et al., “Fiber Optic Cables,    Fundamentals Cable Design System Planning”, 4th revised and enlarged    edition, 2001, Publics MCD Corporation Publishing

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

After studying a conventional optical cable, the inventors found out thefollowing problems. Namely, in the tape-slotted type of optical cable,there easily occurs bending in the coated optical fibers during themanufacture and service, and the micro-bending loss tends to increase.On the other hand, the loose-tube type of optical cable is inferior tothe tape-slotted type of optical cable in respect of the containedefficiency of the coated optical fibers. In this respect, it isconsidered that even in the loose-tube type of optical cable, thecontained efficiency of the coated optical fibers can be improved suchthat an outer diameter of the cable is reduced while the number of thecoated optical fibers to be contained is maintained. However, in thatcase, there occurs a problem that the transmission loss of the coatedoptical fiber may increase.

The present invention is made to solve the aforementioned problem, andit is an object to provide an optical cable having a construction thatenables reduction of an outer diameter of the cable, and/or improvementof contained efficiency of coated optical fibers while an increase oftransmission loss in each of coated optical fibers to be bundled issuppressed.

Means for Solving Problem

In order to overcome the aforementioned problem, an optical cableaccording to the present invention comprises: a tension member; aplurality of tubes each containing a plurality of coated optical fibersand stranded together around the tension member; and an outer sheathcovering the outer periphery of the plurality of tubes stranded togetheraround the tension member.

In particular, a ratio of A/B is 6.3 or more but 7.0 or less, where amode field diameter A of the coated optical fiber falls in a range of8.6±0.4 μm at a wavelength of 1.31 μm, and a fiber cutoff wavelength ofthe coated optical fiber is B μm.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

Effect of the Invention

In accordance with an optical cable of the present invention, reductionof an outer diameter of the cable and/or improvement of containedefficiency of coated optical fibers can be achieved while an increase oftransmission loss in the coated optical fibers is suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic construction of an opticaltransmission system in which one embodiment of an optical cableaccording to the present invention is applied as an optical transmissionline;

FIG. 2 is a view showing a cross-sectional structure of one embodimentof the optical cable according to the present invention (correspondingto the cross section along a line I-I in FIG. 1);

FIG. 3 is a side view showing a construction of one embodiment of theoptical cable according to the present invention (a state that an outersheath is removed);

FIG. 4 is a side view showing a construction of one embodiment of theoptical cable according to the present invention (a state that strandingof a tube is loosened);

FIG. 5 is a side view showing a contained state of the coated opticalfibers contained in each tube in one embodiment of the optical cableaccording to the present invention;

FIG. 6 is a graph indicating bending loss characteristics of a sampleprepared as a coated optical fiber applied to the optical cableaccording to the present invention;

FIG. 7 is a graph indicating transmission loss characteristics of asample prepared as a coated optical fiber applied to the optical cableaccording to the present invention;

FIG. 8 is a graph indicating changes of the transmission loss before andafter cabling of a sample prepared as a coated optical fiber applied tothe optical cable according to the present invention;

FIG. 9 is a graph indicating the change of the transmission loss at awavelength of 1.55 μm in a temperature cycling test of a sample preparedas a coated optical fiber applied to the optical cable according to thepresent invention; and

FIG. 10 is a view for explaining a force-feeding method of the opticalcable according to the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

1 . . . optical transmission system; 10 . . . optical cable; 11 . . .coated optical fiber; 12 . . . tube; 13 . . . tension member; 14 . . .pressure roll; 15 . . . outer sheath; 20 . . . optical transmitter; 30 .. . optical receiver; 110 . . . ribbon fiber; 120 . . . colored thread;200 . . . cable force-feeding dram; and 300 . . . duct.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, embodiments of the optical cable according to thepresent invention will be explained in detail with reference to FIGS. 1to 10. In the explanation of the drawings, the same elements will bedenoted by the same reference symbols and these redundant descriptionswill be omitted.

FIG. 1 is a diagram showing a schematic construction of an opticaltransmission system in which one embodiment of an optical cableaccording to the present invention is applied as an optical transmissionline. An optical transmission system shown in FIG. 1 composes an opticalcable 10, optical transmitters 20 ₁ to 20 _(N), and optical receivers 30₁ to 30 _(N). In the optical cable 10, N optical fibers 11 ₁ to 11 _(N)(N: an integer of two or more) are bundled. The optical transmitters 20₁ to 20 _(K) and the optical receivers 30 _(K+1) to 30 _(N) are arrangedon one end side of the optical cable 10, while the optical transmitters20 _(K+1) to 20 _(N) and the optical receivers 30 ₁ to 30 _(K) arearranged on the other end side of the optical cable 10. The opticaltransmitter 20 _(n) and the optical receiver 30 _(n) are connectedthrough a coated optical fiber 11 _(n) to each other, and opticalsignals transmitted from the optical transmitter 20 _(n) are arrived atthe optical receiver 30 _(n) after propagating the coated optical fiber11 _(n) (n: an integer of 1 or more but N more less).

FIG. 2 is a view showing a cross-sectional structure of the opticalcable according to the present invention, and corresponds to the crosssection along a line I-I in FIG. 1. This optical cable 10 has aloose-tube type of structure such that six tubes 12 ₁ to 12 ₆ arestranded around a tension member 13. Then, these tubes 12 ₁ to 12 ₆ arecovered with a pressure roll 14 and an outer sheath 15. One or morecoated optical fibers 11 (referred to as simply ‘coated optical fiber11’ where an arbitrary one of the plurality of coated optical fibers 11₁ to 11 _(N) is represented) are contained in each tube 12 (referred toas simply ‘tube 12’ where an arbitrary one of the plurality of tubes 12₁ to 12 ₆ is represented). Each coated optical fiber 11 has a mode fielddiameter (MFD) A within the range of 8.6±0.4 μm at a wavelength of 1.31μm, and a ratio A/B is 6.3 or more but 7.0 or less when a fiber cutoffwavelength is set to B μm. More preferably, the ratio A/B is 6.3 or morebut 6.8 or less. Alternatively, in the optical cable 10, a bending lossof each coated optical fiber 11 in the diameter of 20 mm at a wavelengthof 1.55 μm is 3 dB/m or less; more preferably the bending loss is 1.5dB/m or less. For example, the mode field diameter at the wavelength of1.31 μm is 8.53 μm; the fiber cutoff wavelength is 1.3 μm; the ratio A/Bis 6.56; and the bending loss in the diameter of 20 mm is 1.0 dB/m. Theoptical cable 10 thus designed enables reduction of a cable outerdiameter D and/or improvement of contained efficiency of the coatedoptical fibers while suppressing the increase of the transmission lossin each coated optical fiber 11.

More preferably, each coated optical fiber 11 has a transmission loss of0.31 dB/km or less at the wavelength of 1.31 μm, a transmission loss of0.29 dB/km or less at the wavelength of 1.38 μm, and a transmission lossof 0.18 dB/km or less at a wavelength of 1.55 μm. The increase of thetransmission loss at the wavelength of 1.55 μm of each coated opticalfiber 11 is preferably 0.05 dB/km or less after a temperature cycle testwithin a range of temperature from −40° C. to +70° C. Each coatedoptical fiber 11 is placed over four days in an atmosphere of a hydrogenconcentration of 1%. After hydrogen molecules are further removed, theincrease of the transmission loss at the wavelength of 1.38 μm ispreferably 0.05 dB/km or less. After γ-rays of an absorbed dose of 1000Gy/hr are irradiated to each coated optical fiber for an hour, theincrease of the transmission loss of each coated optical fiber 11 at thewavelength of 1.55 μm is preferably 2 dB/km or less.

The thickness of each tube 12 is preferably 0.2 mm or less.Additionally, the coefficient of dynamic friction of the outer sheath 15is preferably 0.30 or less.

The occupied factor of the coated optical fibers 11 inside each tube 12is preferably 20% or more but 75% or less. Here, the occupied factor ofthe coated optical fibers 11 is defined by (sectional area of the coatedoptical fiber 11× the number of fibers)/(sectional area of the tube 12).However, when the occupied factor of the coated optical fibers 11 issmaller than 20%, the outer diameter of the optical cable 10 becomeslarger. Also, when the occupied factor of the coated optical fibers 11is larger than 75%, the transmission loss becomes larger; particularly,the increase of the transmission loss due to cable construction becomeslarger.

A ratio (D/N) is 0.15 mm or less, where D is the outer diameter of theoptical cable 10, and N is the total number of the coated optical fibers11 contained in the optical cable 10. However, when this ratio (D/N) islarger than 0.15 mm, the outer diameter of the optical cable 10 becomeslarger.

FIGS. 3 and 4 are side views each showing the construction of theoptical cable 10. FIGS. 3 and 4 each show a state of the optical cable10 in which the outer sheath 15 and pressure roll 14 are removed over acertain range in the longitudinal direction of the optical fiber 10. Inparticular, FIG. 4 shows a state in which the strand of the tube 12 ₆ isloosened.

As shown in FIG. 3, six tubes 12 ₁ to 12 ₆ are stranded together aroundthe tension member 13. The stranded pitch of each tube 12 around thetension member 13 is preferably 100 m or less. Further, the strandeddirection of each tube 12 around the tension member 13 is preferablyreversed at a given position P in the longitudinal direction of theoptical cable 10. Note that when the stranded pitch of each tube 12around the tension member 13 is larger than 100 mm, temperaturecharacteristics and mechanical characteristics thereof may bedeteriorated.

As shown in FIG. 4, when the outer sheath 15 and pressure roll 14 areremoved over the range L₁ of 500 mm in the longitudinal direction of theoptical cable 10, the length L₂ of the removable tube 12 in which thecoated optical fibers 11 are contained is preferably 20 mm or more. Forthis, the stranded direction of each tube 12 around the tension member13 is reversed at a predetermined position P in the longitudinaldirection of the optical cable 10, and thereby the pitch of the reversedposition P is preferably 500 mm or less. With this construction, in astate where the range in the longitudinal direction of the optical cable10 is suppressed 500 mm or less with respect to the outer sheath 15 andpressure roll 14 to be removed, the coated optical fibers 11 each havinga length required for branching can be taken out.

FIG. 5 is a side view showing a contained state of each coated opticalfiber contained in each tube 12 in the optical cable 10. The area (a) ofFIG. 5 shows a state in which the tube 12 is removed over a certainrange in the longitudinal direction of the optical cable 10. As shown inFIG. 5, a plurality of coated optical fibers 11 in each tube 12 arecontained, and the plurality of coated optical fibers 11 are strandedtogether with each other. The stranded direction of the plurality ofcoated optical fibers 11 may be reversed at a predetermined position inthe longitudinal direction of the optical cable 10. Also, the pluralityof coated optical fibers 11 are divided into a plurality of groups, andthe coated optical fibers 11 of each group may be bundled by a coloredthread.

In addition, as shown in the area (b) of FIG. 5, the coated opticalfibers 11 may be contained in each tube 12 in a state of one or moreribbon fibers 110. Further, as shown in the area (c) of FIG. 5, thecoated optical fibers 11 contained in each tube 12 may be divided into aplurality of groups, and the coated optical fibers 11 of each group maybe bundled by colored threads 120.

An extra length ratio of each coated optical fiber 11 to each tube 12 ispreferably 0% or more but 0.10% or less. In this case, the extra lengthratio of each coated optical fiber 11 is defined by (100%×(coatedoptical fiber length−tube length))/(tube length)). Further, when theextra length ratio is smaller than 0%, the transmission loss at a hightemperature (e.g., 70° C. or more) is boosted, while when the extralength ratio is larger than 0.10%, the transmission loss even at a lowtemperature (e.g., −40° C. or less) is increased.

On the other hand, when a cable is used in such an extremely colddistrict that the lowest temperature can reach −50° C. or −60° C., theextra length ratio in an initial state is desirably −0.03% or more but0% or less.

In accordance with the optical cable 10, even in a linear cable, thereoccurs some bending in each coated optical fiber 11 due to stranding ofeach tube 12 and an extra length and/or stranding provided at eachcoated optical fiber 11. In this case, the minimum radius of curvatureof each coated optical fiber 11 is preferably 15 mm or more but 100 mmor less.

It is more preferable that the thus constructed optical cable 10 has thefollowing constructions or characteristics. That is, a ratio (W/N) ispreferably 0.7 kg/km or less, where W is the cable weight per unitlength of the optical cable 10, and N is the total number of the coatedoptical fibers 11 contained in the optical cable 10. When a testaccording to various types of mechanical test methods prescribed inTelecordia GR-20 Section 6.5 is carried out, an increase in transmissionloss at the wavelength of 1.55 μm of each coated optical fiber 11 ispreferably 0.05 dB or less during and after the test. PMDq of a coatedoptical fiber according to a test method prescribed in Section 5.5 andAnnex A of IEC 60794-3 is preferably 0.05 ps/km^(1/2) or less.

In addition, the bending rigidity of the optical cable 10 is preferably5000 kg·mm² or more but 15000 kg·mm² or less. However, assuming that thebending loss of the optical cable 10 is smaller than 5000 kg·mm², theoptical cable 10 can jam on the way of a duct when the optical cable 10is underlaid by feeding by force within the duct, which makes itimpossible to underlay the optical cable 10. On the other hand, assumingthat the bending rigidity of the optical cable 10 is larger than 15000kg·mm², the optical cable 10 cannot pass through a complicated duct whenthe optical cable 10 is underlaid by feeding by force within the duct,which still makes it impossible to underlay the optical cable 10.Similarly, the coefficient of dynamic friction of a material of an outersheath of the cable is desirably 0.30 or less. Thus, in the case wherethe optical cable 10 is underlaid within the duct by feeding by force,the force-feeding rate is preferably 20 m/min or more in view ofunderlaying time and labor costs.

Next, an explanation will be given of a specific sample prepared for theoptical cable 10 according to the aforementioned embodiment. In theoptical cable 10 of a prepared sample, an outer diameter of each coatedoptical fiber 11 is 0.25 mm, an inner diameter of each tube 12 is 1.2mm, outer diameters of each tube 12 and the tension member 13 arerespectively 1.5 mm, and a cable outer diameter D is 6.7 mm. Twelvecoated optical fibers 11 within each tube 12 are contained, and thetotal number N of the coated optical fibers 11 contained in the opticalcable 10 is seventy-two. The six tubes 12 are stranded together aroundthe tension member 13, the stranded pitch is 70 mm, and the strandeddirection is reversed at a predetermined position in the longitudinaldirection of the optical cable 10. Note that a pitch at the reversedposition is 420 mm. Each tube 12 is made of polybutylene terephthalate,while the outer sheath is made of polyethylene. A cable weight per unitlength is 42 kg/km.

Each coated optical fiber 11 composes a core region made of pure silicaglass, and a cladding region made of F-doped silica glass provided atthe outer periphery of the core region. The core region has an outerdiameter (core diameter) of 7.9 μm, and a relative refractive indexdifference of 0.39% with respect to the cladding region. Each coatedoptical fiber 11 having such a structure has the following variouscharacteristics. That is, a mode field diameter A at the wavelength of1.31 μm was 8.53 μm, a fiber cutoff wavelength B was 1.23 μm, a ratioA/B was 6.93, and the wavelength of zero dispersion was 1.318 μm. Thetransmission loss at the wavelength of 1.31 μm was 0.289 dB/km or less,the transmission loss at the wavelength of 1.383 μm was 0.247 dB/km orless, and the transmission loss at the wavelength of 1.55 μm was 0.174dB/km or less. Further, for several characteristics at the wavelength of1.55 μm, the bending loss in the diameter of 20 mm was 1.8 dB/m, thechromatic dispersion was 15.0 ps/nm/km, the dispersion slope was 0.054ps/nm²/km, and RDS (=dispersion slope/chromatic dispersion) was 0.0036/nm. The dispersion slope at a zero dispersion wavelength was 0.079ps/nm²/km. The polarization mode dispersion was 0.03 ps/km^(1/2).

FIG. 6 is a graph indicating bending loss characteristics of a sampleprepared as a coated optical fiber 11. Also, FIG. 7 is a graphindicating transmission loss characteristics of the sample prepared as acoated optical fiber 11. As a comparative example for these graphs,characteristics of an optical fiber in compliance with the standard ofITU-TG.652.D are indicated by broken lines. In addition, in FIG. 6,graphs G601 and G602 indicate the bending losses of the sample and thecomparative example, respectively. In FIG. 7, graphs G701 and G702indicate the transmission losses of the sample and the comparativeexample, respectively.

As is apparent from FIGS. 6 and 7, the sample prepared as a coatedoptical fiber 11 is excellent in both the bending loss characteristicand transmission loss characteristic as compared to the optical fiber ofthe comparative example. Therefore, the coated optical fiber of thesample is applicable to a center-core type of optical cable, for anothercable construction, in which a loose tube is arranged in the center ofthe cable while a tension member and/or inclusions are arranged on theperiphery thereof.

An optical cable applied with the coated optical fibers each having theaforementioned characteristics (the above sample) is manufactured, and avariety of tests are implemented to this optical cable.

FIG. 8 is a graph indicating changes before and after cabling of thetransmission loss of the above sample prepared as a coated optical fiber11. Note that in FIG. 8, graph G801, graph G802, and graph G803 indicatethe transmission losses at the wavelengths of 1.31 μm, 1.55 μm and 1.625μm, respectively.

As is apparently understood from FIG. 8, an increase of the transmissionloss in the sample caused by the cabling is not seen.

In addition, a temperature cycling test within the temperature rangefrom −40° C. to +70° C. is implemented to an optical cable having acable length of 1 km, applied with the coated optical fibers of theabove sample. FIG. 9 is a graph indicating the change of thetransmission loss at the wavelength of 1.55 μm in the temperaturecycling test of the above sample prepared as a coated optical fiber. Asis seen from FIG. 9, it is ascertained that the increase of thetransmission loss of the sample at the wavelength of 1.55 μm issuppressed to 0.01 dB/km at the maximum. Thus, it is confirmed that theoptical fiber applied as a coated optical fiber of the above sample hasexcellent temperature characteristics.

Further, a variety of mechanical tests according to test methodsprescribed in Telecordia GR-20 to the optical fiber were implemented,applied as a coated optical fiber of the above sample. In a test pullingan optical cable having a cable length of 5.5 m at a tension strength of1100 N, an increase of the transmission loss was 0.01 dB or less. In atest in which wrenching of ±90° was applied five times in a state whereone end of an optical cable having a cable length of 1 m was fixed,while a load of 44 N was applied at the other end thereof, an increaseof the transmission loss was 0.01 dB or less. In a test that bending of360° was applied twenty-five times to an optical cable having a cablelength of 5 m in a bending diameter of 160 mm, an increase of thetransmission loss was 0.01 dB or less. In a test that a side pressure(2200 N in an initial period and 1100 N in a long period) was appliedover the range of 100 mm in the longitudinal direction of an opticalcable, an increase of the transmission loss was 0.03 dB or less. Also,in a test that an impact having an energy of 5 J was applied to theoptical cable when an object was dropped from a position of 1 m inheight, an increase of the transmission loss was 0.01 dB or less. Asmentioned above, in each test prescribed in Telecordia GR-20, it wasascertained that the increase of the transmission loss in the opticalcable was smaller than an allowable value. Thus, it was confirmed thatthe optical cable applied with the coated optical fibers of the abovesample had an excellent mechanical characteristic.

As described above, while the optical cable 10 according to the presentinvention, as compared to a conventional seventy-two-fiber opticalcable, has the characteristics equal to the conventional one, a diameterof the cable is reduced by 40%, and a sectional area of the cableperforms substantially one-third, which enables a sufficiently reduceddiameter. Alternatively, while the optical cable 10 according to thepresent invention, as compared to the conventional optical cable, hasthe characteristics equal to the conventional one, the containedefficiency of the coated optical fiber can be improved.

Furthermore, the optical cable 10 according to the present invention isapplicable to a cable for microduct-force-feeding while making use oflight weight and flexibility with development of reduced diameter. Inparticular, as shown in FIG. 10, the optical cable 10 having apredetermined length is rolled around a force-feeding drum 200, and theoptical cable 10 is force-fed from this drum 200 into a duct 300 at aforce-feeding rate of 20 m/min or more. In this way, an underlay of theoptical cable 10 is carried out into an existing duct. Here, FIG. 10 isa view for explaining a force-feeding method of an optical cableaccording to the present invention.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

INDUSTRIAL APPLICABILITY

The optical cable according to the present invention is preferablyapplicable to an optical transmission line for an optical transmissionsystem with a large capacity and so on.

1. An optical cable, comprising: a tension member; a tube comprised ofone of plastic and metal, stranded together around said tension member,and containing one or more coated optical fibers inside; and an outersheath covering an outer periphery of said tube, wherein a ratio of A/Bis 6.3 or more but 7.0 or less, where said each coated optical fiber hasa mode field diameter A of 8.6±0.4 μm at a wavelength of 1.31 μm, and afiber cutoff wavelength of said each coated optical fiber is B μm, andwherein each of said coated optical fibers comprises a core region madeof pure silica glass, and a cladding region made of F-doped silicaglass.
 2. An optical cable according to claim 1, wherein a bending lossof said each coated optical fiber in the diameter of 20 mm at awavelength of 1.55 μm is 3 dB/m or less.
 3. An optical cable accordingto claim 1, wherein an extra length ratio of said each coated opticalfiber to said tube is more than 0% but 0.10% or less.
 4. An opticalcable according to claim 1, wherein an extra length ratio of said eachcoated optical fibers to said tube is −0.03% or more but less than 0%.5. An optical cable according to claim 1, wherein an occupied factor ofsaid coated optical fibers within said tube is 20% or more but 75% orless.
 6. An optical transmission system comprising an optical cableaccording to claim 1 for an optical transmission line for transmittingoptical signals.
 7. A force-feeding method, comprising the steps of:preparing an optical cable according to claim 1; and force-feeding saidprepared optical cable at a force-feeding rate of 20 m/min or more. 8.An optical cable, comprising: a tension member; a tube comprised of oneof plastic and metal, stranded together around said tension member, andcontaining one or more coated optical fibers; and an outer sheathcovering an outer periphery of said tube, wherein each of said coatedoptical fibers comprises a core region made of pure silica glass, and acladding region made of F-doped silica glass.
 9. An optical cableaccording to claim 8, wherein a bending loss of said each coated opticalfiber in the diameter of 20 mm at a wavelength of 1.55 μm is 3 dB/m orless.
 10. An optical cable according to claim 8, wherein said eachcoated optical fiber has a transmission loss of 0.31 dB/km or less at awavelength of 1.31 μm, a transmission loss of 0.29 dB/km or less at awavelength of 1.38 μm, and a transmission loss of 0.18 dB/km or less ata wavelength of 1.55 μm.
 11. An optical cable according to claim 8,wherein an increase of said each coated optical fiber is 0.05 dB/km orless at a wavelength of 1.38 μm after said each coated optical fiber isplaced over four days in an atmosphere of a hydrogen concentration of 1%and then hydrogen molecules are removed.
 12. An optical cable accordingto claim 8, wherein an increase of said each coated optical fiber is 2dB/km or less at a wavelength of 1.55 μm after said each coated opticalfiber is irradiated for an hour by γ rays of an absorbed dose of 1000Gy/hr.
 13. An optical cable according to claim 8, wherein an extralength ratio of said each coated optical fiber to said tube is more than0% but 0.10% or less.
 14. An optical cable according to claim 8, whereinan extra length ratio of said each coated optical fibers to said tube is−0.03% or more but less than 0%.
 15. An optical cable according to claim8, wherein an occupied factor of said coated optical fibers within saidtube is 20% or more but 75% or less.